Maedi visna (MV) and caprine arthritis-encephalitis (CAE) are two retroviral infections distributed world wide. Antigenic cross reactions between the viruses have been demonstrated in gag and env encoded structural proteins. Antigens from ovine lentiviruses are easier to produce in cell culture systems and therefore have been used in the development of diagnostic tests for both infections. Antigenically relevant epitopes have been characterised in the transmembrane protein, but little information is available on the immunodominant and cross reacting epitopes in the major capsid antigen (p25). In this study four different recombinant subunits of ovine lentivirus p25 were tested against sera from infected goats and a detailed characterisation of the immunodominant subunit was carried out. Highest ELISA absorbances were obtained with a 29 amino acid subunit located in the N'-terminal half of p25. Through the analysis of overlapping peptides spanning this region we identified a 17 amino acid sequence that can be used in the development of a highly standardized synthetic peptide-based assay.